MQTT 5 for Kotlin Multiplatform, split into:

• mktt-core: protocol types, properties, packets, codecs
• mktt-client: coroutine and Flow based client built on top of mktt-core

dependencies {
    implementation("io.github.nicolasfara:mktt-core:<version>")
    implementation("io.github.nicolasfara:mktt-client:<version>")
}

import io.github.nicolasfara.mktt.client.MqttClient
import io.github.nicolasfara.mktt.client.PublishRequest
import io.github.nicolasfara.mktt.core.QoS
import io.github.nicolasfara.mktt.core.Topic
import io.github.nicolasfara.mktt.core.TopicFilter
import kotlinx.coroutines.flow.first

suspend fun sample() {
    val client = MqttClient("localhost", 1883) {
        clientId = "mktt-sample"
    }

    client.connect()
    client.subscribe(listOf(TopicFilter(Topic("sample/topic"))))

    client.publish(
        PublishRequest("sample/topic") {
            desiredQoS = QoS.AT_LEAST_ONCE
            payload("hello")
        },
    )

    val message = client.messages(TopicFilter(Topic("sample/topic"))).first()
    println(message.payloadAsString())

    client.disconnect()
    client.close()
}

mktt-core exposes:

• MQTT 5 reason codes and properties
• topic and subscription models
• packet definitions for CONNECT, CONNACK, PUBLISH, PUBACK/PUBREC/PUBREL/PUBCOMP, SUBSCRIBE, UNSUBSCRIBE, PING and DISCONNECT
• packet encode/decode utilities over Ktor channels and kotlinx-io

mktt-client exposes:

• MqttClient
• StateFlow<MqttConnectionState>
• SharedFlow<MqttPublishMessage>
• messages(filter) local filtering helper
• coroutine-based connect, publish, subscribe, unsubscribe, and disconnect

• mktt-core is covered by deterministic packet/property round-trip tests.
• mktt-client keeps fast unit tests in commonTest/jvmTest and runs broker-based checks in jvmIntegrationTest.

Run unit tests:

./gradlew unitTest

Run integration tests against the local Mosquitto container scaffold (mktt-client/src/jvmIntegrationTest/resources):

MKTT_RUN_INTEGRATION_TESTS=true ./gradlew integrationTest

Optional public broker test (opt-in only):

MKTT_RUN_INTEGRATION_TESTS=true MKTT_RUN_REMOTE_BROKER_TESTS=true ./gradlew integrationTest